Device for optical signal processing showing transistor operation

ABSTRACT

An optical device provided with a junction region for reflecting radiation to be transmitted in the device, and a gate for controlling the electron density in said junction region, is fast and simple to manufacture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for optical signal processing.

It is an object of the present invention to provide a device for opticalsignal processing, with which the magnitude of a flow of light or alightbeam in semiconductor can be influenced in a simple way.

2. Description of the Background Art

Known devices for optical signal processing make use of the non-lineareffect; the disadvantages thereof are the requirement of a rather highlevel of power or slowness. Fabry-Perot resonators or resonators usingsuperlattices are required for this purpose; these are difficult tointegrate.

Further there is known a device showing distributed feedback transistoroperation, in which a lattice is employed. This device is also based onnon-linear effect.

Further the following prior art is known: Journal of LightwaveTechnology, vol. LT-5, No. 9, September 1987, IEEE, J. H. Abeles et al.:"Novel single quantum well optoelectronic devices based on excitonbleaching", pages 1296-1300; Electronics Letters, Vol. 22, No. 18, Aug.28, 1986, S. H. Lin et al.: "GaAs PIN electro-optic travelling-wavemodulator at 1.3 μm", pages 934-935; Applied Physics Letters, vol. 48,No. 19, May 12, 1986, American Institute of Physics, A. Alping et al.:"Highly efficient waveguide phase modulator for integratedoptoelectronics, pages 1243-1245; EP-A-0209190; Applied Physics Letters,vol. 50, No. 15, Apr. 13, 1987, T. Hiroshima: "Electric field inducedrefractive index changes in GaAs-Al_(x) Ga_(1-x) As quantum wells",pages 968-870; EP-A-0233011; Journal of Lightwave Technology, vol. LT-5,No. 10, October 1987, R. G. Walker: "High-speed electrooptic modulationin GaAs/GaAlAs waveguide devices", pages 1444- 1453.

A further object of the present invention is to provide a device withwhich switching rates in the order of picoseconds or subpicoseconds canbe achieved.

SUMMARY OF THE INVENTION

The foregoing and other objectives are achieved by the present inventionof a device for optical signal processing comprising a substrate, awaveguide of doped semiconductor material on top of the substrate, thewaveguide having a refractive index which is not greater than therefractive index of the substrate, and with a junction region betweenthe waveguide and the substrate. A gate for receiving an electricpotential with respect to the substrate is positioned on top of thewaveguide.

The materials chosen for the substrate and the waveguide are selectedsuch that there is a discontinuity between their conducting bands,causing electrons in the waveguide to migrate to the substrate, therebycreating a two-dimensional gas of electrons having a very high mobilityin the junction region. When light is passed through the waveguide in adirection parallel to the substrate, the passage of the light can becontrolled by selectively applying a voltage to the gate which causesthe electron gas to be depleted in the junction region. The frequency ofthe light is preferably lower than the plasma frequency of the electrongas.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics, details and advantages of the present inventionwill be clear in view of the following description of preferredembodiments of the device according to the present invention,illustrated by means of a drawing, in which show:

FIG. 1 a diagramatic sectional view of the structure of a firstembodiment;

FIG. 2 a diagram of the bands of the structure from FIG. 1;

FIG. 3 a diagram of the structure according to a second preferredembodiment;

FIG. 4 a graph in which the absorption and reflection coefficients areplotted relative to the densitiy of carriers for the TE mode of thestructure from FIG. 3;

FIG. 5 a graph similar to FIG. 4 for the TM mode of the structure fromFIG. 3;

FIG. 6 a diagramatic sectional view of a third preferred embodiment ofthe device according to the present invention;

FIG. 7 a perspective view of the embodiment from FIG. 6;

FIG. 8 a graph in which absorption and reflection coefficients areplotted as function of the density of carriers in the structure of FIG.6, relating to different angles of incidence;

FIG. 9 a sectional view of a fourth preferred embodiment of the deviceaccording to the present invention; and

FIG. 10 a diagramatic top view of the structure from FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A layer of AlGaAs:Si₂, e.g. having a dope of 3×10¹⁸ silicon atoms percm², is disposed on a substrate 1 of GaAs (FIG. 1), whereby a junctionregion 3 is created into the GaAs substrate. Due to a discontinuityexisting between the conducting bands of GaAs and AlGaAs the electronsmigrate from the AlGaAs material to the GaAs substrate, whereby atwo-dimensional "gas" of electrons having a very high mobility (HEMT) iscreated into the junction region 3 of the GaAs substrate 1, borderingthe AlGaAs layer. A gate 4 to be provided with a conducting terminal isdisposed onto the layer or sheet AlGaAs:Si.

When no voltage is applied to gate 4, a light beam may be sent ortransmitted through the channel region 2 according to arrows L (in FIG.1 from the left side [source] to the right side [drain]), which willpass unobstructedly, as the "electron gas" 3 is a perfect mirror orreflecting surface for this light under certain circumstances. It isimportant herewith that the electron density in the junction region 3 isas large as possible. Further it is of importance that the plasmafrequency of the electron gas is higher than the frequency of the usedlight. The mobility of the electrons should be high enough to preventdissipation of the beam in the electron gas; selectively doped heterojunctions provide the solution (transistor operation or light modulatingoperation).

By applying a voltage to gate 4, the electron gas is depleted in therequired region 3 under the gate 4 whereby the reflection properties ofthe junction region 3 decrease and the light magnitude from source tothe drain also decreases. Accordingly a transistor operation for lightis obtained through gate 4, corresponding to a field effect transistor.

FIG. 2 shows the bands of the structure of FIG. 1, wherein E_(C) is thelevel of the conduction band and E_(F) is the Fermi level.

In a second embodiment (FIG. 3) a layer 6 e.g. of undoped Al₀.₃ Ga₀.₇ Asof 1-10 μm thickness (in which the dissipation of free electrons fromdonors is prevented) includes a doped part 7 of AlGaAs:Si (e.g. 3.10₁₈cm⁻³) of 60 nm thickness, whereby a diagramatically depicted region 9 iscreated into the semiconducting GaAs substrate 8, in which thereflecting electron gas is built up.

When light is sent through layer 6, the structure of FIG. 3 shows anabsorption and reflection coefficient as a function of different wavelengths, being different for different polarization modes of theincident light (FIG. 4: TE mode, FIG. 5: TM mode). In both graphs thecurves starting at the left underside show the reflection coefficient,while the absorption coefficient is given by curves extending from theleft upperside to the right upperside. The distance or interval betweenthe reflection and absorption curves determines the transmission for thestructure of FIG. 3. The curves of FIG. 4 and 5 relate to curvescomputed on the basis of the Maxwell equations and the plasmadynamicalproperties of the electron gas.

In the TE mode electron concentrations of 10²⁰ cm⁻³ are required forobtaining a sufficient high reflection at a wave length of 10 μm. Forshorter wavelengths higher concentrations are required. In the TM mode apeak in the R-value of 5.10¹⁸ cm⁻³ exists at 10 μm. The concentrationsapproach the maximum that is obtainable at a junction surface betweenAlGaAs and GaAs. Illumination of the complete structure by a source ofthe required wave length (photonenergy higher than the band gap of e.g.GaAs) will excite deep donor levels and create electron hole pairs,whereby the electron density (in the channel) increases. Therefore itbecomes possible to modulate light having a lower wavelength than 10 μm.

Further semiconductors can be used, in which electrons have a smallereffective mass. Heterojunctions between e.g. CdTe and InSb of a forcedlayer of GaAs-GaInSb combine higher electron densities with a smallereffective electron mass; the plasma frequency and the reflectioncoefficient are increased; the mobility is higher; the dissipation isdecreased.

A further optimized structure (FIGS. 6, 7), comprises a undoped GaAssubstrate 11, a layer 12 AlAs:Si of 60 nm thickness, a layer 13 of GaAsof 30 nm thickness, a layer 14 AlAs:Si of 60 nm thickness disposedthereon and a wave guide 15 thereupon of undoped GaAs. Between layers 11and 12 a diagrammatically shown electron gas 16 of e.g. 10 nm thicknessis created, while in layer 13 an electron gas of e.g. 30 nm thicknessand in the GaAs wave guide an electron gas 17 of 10 nm thickness iscreated. On the wave guide a control gate 18 is disposed.

Such a double hetero junction structure is discribed in the article"Physical limits of heterostructure field-effect transistors andpossiblities of novel quantum field-effect devices", IEEE J. of Qua.El., vol. QE-22, nr. 9, pag. 1845-1852, 1986 of H. Sakaki. The discloseddouble heterojunction structure provides a much thicker electron gasthan as above described. Herewith the reflection coefficient increasesand the absorption (dissipation) coefficient for the TE and TM mode (inthe TM mode the reflection peak is wider, whereby the component will beless sensitive to fluctuations in electron density) decreases. Bystacking a plurality of these double hetero junctions the reflectionproperties are increased. The modulation depth for the light isdetermined by the maximal thickness of a depleted layer to be created bya gate voltage; material parameters like doping and dielectricalconstant have an influence thereto, e.g. at a selectively dopedGaAs-AlGaAs double heterojunction (3.10¹⁸ cm⁻³), the thickness is e.g.approximately˜50 nm. At minimal gate length only one doubleheterojunction is used (the total thickness of the electron gas in sucha structure is approximately 50 nm). When a plurality of layers isstacked upon one another and a complete modulation of the light in thechannel is to be obtained, the gate length is to be increased(additional resiprositation pass of the light beam in the light guide,dependent on the number of additional double heterojunctions).

The relation between the angle of incidence and the steepness of thereflection curve (FIG. 8) offers the possibility of obtaining atransistor operation having a high "transconductance", viz. a largechange in light intensity divided by the change of gate voltage, as anincrease of the electron density of approximately 50% forces the devicefrom "OFF" to "ON". In the structure shown in FIG. 4 two of the threeelectron gasses extend also outside the gate region. The switching rateis determined by the smallest of the two dimensions of the gate, viz.length and width. The chosen angle of incidence determined a minimumvalue to the gate length (e.g. for 10 μm wave length and a angle ofincidents of 89° the minimal gate length is 0.5 mm); short switchingtimes are obtained by decreasing the width. At light of a wave length of10 μm a light wave (of GaAs) of 10 μm high and 1.5 μm wide(theoretically) may operate at a clock frequency of 100 GHz-1 THz,dependent on the chosen angle of incidence and therefore on the numberof electrons to be displaced. The switching rate will be determined bythe delay in the transmission line to the electrical gate.

In a fourth embodiment of the present invention (FIGS. 9, 10), a channelregion is provided with a gate region 26 of which a control region 27 isa part. This control region is doped with donors over a thickness ofe.g. 60 nm. The first layer of e.g. 10 nm thickness from the junctionregion is undoped and reflects the resulting electron layer in aninsufficient way due to insufficient density.

When a light beam is sent through gate region 26, a region 28 of thejunction region 23 will obtain a sufficient reflection coefficient bymeans of this light beam, whereby the light beam serves as gate signal.It is also possible that the electron layer reflects in a sufficientway; an additional amount of electrons will be transported through thegate region by means of a light beam to the already existing electrongas, whereby the reflection diminishes in the TM mode. This second caseoperates in an inverting way. As is to be seen in FIG. 10, a gate lightbeam (arrow G) may control more than one gate region 26, when this lightbeam G has a sufficiently high magnitude and is coupled to acorresponding sequential gate region 26 in a not shown way.

In another possible embodiment for completely optical switching there ismade use of the low value in the band in the upper conduction band ofGaAs or InSb. When e.g. in GaAs a light beam of 0.31 eV wavelength orsmaller is used, electrons will be transferred to the upper low value,whereby the plasma frequency is divided by a factor 8. The reflectioncoefficient drops hereby to a neglectable value. The relaxation time ofthe electrons is very short (approx. 1 psec.) such that a very fast,completely optical circuit is possible. The switching mechanism operatesin an inverting way, whereby logical applications become possible.

A further, completely optical circuit is obtained by using a light beamas gate signal having a wave length similar to or smaller than the bandgap wavelength of the semiconductor containing the electron gas andbeing perpendicularly incident light to the layer structure. As thesemiconductor containing the electrons will have the smallest band gap,a gate signal of a fitting wave length will only create electron holepairs in the reflecting part (e.g. a wave guide of undoped AlAs and asubstrate of GaAs covered with e.g. an undoped AlAs buffer layer waveguide of 1 μm thickness and a buffer separated by a doubleheterojunction between GaAs and AlAs:Si). With the requirement of aminimum of power (μW/μm²), additional electrons can thus be created inthe already existing electron gas, which will switch the transistor"OFF" in the TM mode. This embodiment is also operated invertingly. Asin this embodiment no electrons have to be displaced in space, butswitching takes place through creation and annihilation of electron holepairs, the switching time is only determined by the generation andrecombination time of these pairs; this can be obtained in a few decadesof femto seconds.

The last mentioned design is further improved by stacking a number ofthese heterojunctions (growing of a so-called super lattice). HerewithBragg reflection or constructive interference will be obtained, suchthat the reflection coefficient for the slanting light (in the waveguide) becomes high (˜1) and less light is lost in the channel. As thegate signal (perpendicularly incident) is only absorbed in the electrongas layers it will penetrate through the complete super lattice(contrary to the embodiment having an electronic gate in which themodulation depth is determined by the Debye-length in the semiconductor) and will create electron hole pairs in the material havingthe small band gap. Herewith the refraction index of the material havingthe small bandgap is changed and therefore a deviation is created of theBragg and/or interferance properties, whereby the reflection coefficientdecreases highly.

By using semiconductors having a very small effective mass of theelectrons, shorter wave lengths can be used in the channel, even thesame wave length for the channel and gate signal. If e.g. a structure ofInSb as electron gas carrier (e.g. a heterojunction between the abovementioned CdTe and InSb) is used a channel wave length of 2.4 μm orhigher can be used according to this theory; as the band gap of InSb isonly 0.17 eV (corresponding to a wave length of 7.3 μm), a completelyoptically switching element that may be inverting and may be connectedas cascade may be made using a system based on InSb, and switched withinfsec. Therefore optical information processing having a band width inthe THz range (e.g. optical computers having a THz optical clock) willbe achievable.

Signal processing properties to be obtained by the above device are thefollowing:

high rates may be achieved because an electron gas is used;

using small, non-lineair effects is prevented by conducting light inmaterial having a small refrective index;

power dissipation will be small and the described preferred embodimentswill use known semiconductor techniques.

I claim:
 1. A device for optical signal processing comprising:asubstrate; a waveguide of doped semiconductor material on top of thesubstrate, the waveguide having a refractive index which is not greaterthan the refractive index of the substrate; a junction region betweenthe waveguide and the substrate; and a gate for receiving an electricpotential with respect to the substrate is positioned on top of thewaveguide, whereby the passage of light through the waveguide in adirection parallel to the junction region is controllable by theapplication of an electrical potential to the gate.
 2. A device foroptical signal processing as recited in claim 1, wherein the materialschosen for the substrate and the waveguide are selected such that thereis a discontinuity between their conducting bands, causing electrons inthe waveguide to migrate to the substrate, thereby creating atwo-dimensional gas of electrons having a very high mobility in thejunction region.
 3. A device for optical signal processing as recited inclaim 2, the plasma frequency of the electron gas is preferably higherthan the frequency of the light.
 4. A device for optical signalprocessing as recited in claim 1, wherein the substrate comprises GaAsand the waveguide comprises AlGaAs:Si.
 5. A device for optical signalprocessing as recited in claim 1, wherein the waveguide consists ofnon-doped GaAs, the substrate consists of GaAs, and the junction regioncomprises a double heterojunction structure of GaAs and AlAs:Si.
 6. Adevice for optical signal processing as recited in claim 1, wherein thewaveguide consists of non-doped AlAs, the substrate comprises anon-doped AlAs layer of 1 μm thickness and the junction region betweenthe waveguide and the substrate comprises a double heterojunction ofGaAs and AlAs:Si.
 7. A device for optical signal processing as recitedin claim 1, wherein the waveguide consists of non-doped SdTe, thesubstrate comprises CdTe and the junction region between the waveguideand the substrate comprises a double heterojunction between InSb andSdTe doped with Si.
 8. A device for optical signal processing as recitedin claim 1, wherein the substrate comprises undoped n-type GaAs, thewaveguide undoped p-type GaAs and the junction region comprises anundoped layer of GaAs having a thickness equal to or greater than 100 nmand a doping concentration of 3.10¹⁸ cm⁻³ <N_(D) <1.10¹⁹ cm⁻³, which isdirectly on top of the substrate and a barrier layer on top of thejunction region of approximately 30 nm of undoped AlAs.